The scientists were wrong. They were wrong about Pluto ever being a planet. Pluto got listed as a planet because they were specifically searching for a ninth planet (long story involving miscalculations about Uranus' orbit), so as soon as they found a moving object beyond Neptune, it got stamped as a planet.

If Pluto had been discovered in the 90s, the astronomers would've just listed it as a large Trans-Neptunian Object and left it at that.